Network Working Group M. Belshe
Internet-Draft Twist
Expires: August 4, 2012 R. Peon
Google, Inc
Feb 2012
SPDY Protocoldraft-mbelshe-httpbis-spdy-00
Abstract
This document describes SPDY, a protocol designed for low-latency
transport of content over the World Wide Web. SPDY introduces two
layers of protocol. The lower layer is a general purpose framing
layer which can be used atop a reliable transport (likely TCP) for
multiplexed, prioritized, and compressed data communication of many
concurrent streams. The upper layer of the protocol provides HTTP-
like RFC2616 [RFC2616] semantics for compatibility with existing HTTP
application servers.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 4, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Internet-Draft SPDY Feb 20121. Overview
One of the bottlenecks of HTTP implementations is that HTTP relies on
multiple connections for concurrency. This causes several problems,
including additional round trips for connection setup, slow-start
delays, and connection rationing by the client, where it tries to
avoid opening too many connections to any single server. HTTP
pipelining helps some, but only achieves partial multiplexing. In
addition, pipelining has proven non-deployable in existing browsers
due to intermediary interference.
SPDY adds a framing layer for multiplexing multiple, concurrent
streams across a single TCP connection (or any reliable transport
stream). The framing layer is optimized for HTTP-like request-
response streams, such that applications which run over HTTP today
can work over SPDY with little or no change on behalf of the web
application writer.
The SPDY session offers four improvements over HTTP:
Multiplexed requests: There is no limit to the number of requests
that can be issued concurrently over a single SPDY connection.
Prioritized requests: Clients can request certain resources to be
delivered first. This avoids the problem of congesting the
network channel with non-critical resources when a high-priority
request is pending.
Compressed headers: Clients today send a significant amount of
redundant data in the form of HTTP headers. Because a single web
page may require 50 or 100 subrequests, this data is significant.
Server pushed streams: Server Push enables content to be pushed
from servers to clients without a request.
SPDY attempts to preserve the existing semantics of HTTP. All
features such as cookies, ETags, Vary headers, Content-Encoding
negotiations, etc work as they do with HTTP; SPDY only replaces the
way the data is written to the network.
1.1. Document Organization
The SPDY Specification is split into two parts: a framing layer
(Section 2), which multiplexes a TCP connection into independent,
length-prefixed frames, and an HTTP layer (Section 3), which
specifies the mechanism for overlaying HTTP request/response pairs on
top of the framing layer. While some of the framing layer concepts
are isolated from the HTTP layer, building a generic framing layer
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has not been a goal. The framing layer is tailored to the needs of
the HTTP protocol and server push.
1.2. Definitions
client: The endpoint initiating the SPDY session.
connection: A transport-level connection between two endpoints.
endpoint: Either the client or server of a connection.
frame: A header-prefixed sequence of bytes sent over a SPDY
session.
server: The endpoint which did not initiate the SPDY session.
session: A synonym for a connection.
session error: An error on the SPDY session.
stream: A bi-directional flow of bytes across a virtual channel
within a SPDY session.
stream error: An error on an individual SPDY stream.
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Internet-Draft SPDY Feb 20122. SPDY Framing Layer2.1. Session (Connections)
The SPDY framing layer (or "session") runs atop a reliable transport
layer such as TCP [RFC0793]. The client is the TCP connection
initiator. SPDY connections are persistent connections.
For best performance, it is expected that clients will not close open
connections until the user navigates away from all web pages
referencing a connection, or until the server closes the connection.
Servers are encouraged to leave connections open for as long as
possible, but can terminate idle connections if necessary. When
either endpoint closes the transport-level connection, it MUST first
send a GOAWAY (Section 2.6.6) frame so that the endpoints can
reliably determine if requests finished before the close.
2.2. Framing
Once the connection is established, clients and servers exchange
framed messages. There are two types of frames: control frames
(Section 2.2.1) and data frames (Section 2.2.2). Frames always have
a common header which is 8 bytes in length.
The first bit is a control bit indicating whether a frame is a
control frame or data frame. Control frames carry a version number,
a frame type, flags, and a length. Data frames contain the stream
ID, flags, and the length for the payload carried after the common
header. The simple header is designed to make reading and writing of
frames easy.
All integer values, including length, version, and type, are in
network byte order. SPDY does not enforce alignment of types in
dynamically sized frames.
2.2.1. Control frames
+----------------------------------+
|C| Version(15bits) | Type(16bits) |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Data |
+----------------------------------+
Control bit: The 'C' bit is a single bit indicating if this is a
control message. For control frames this value is always 1.
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Version: The version number of the SPDY protocol. This document
describes SPDY version 3.
Type: The type of control frame. See Control Frames for the complete
list of control frames.
Flags: Flags related to this frame. Flags for control frames and
data frames are different.
Length: An unsigned 24-bit value representing the number of bytes
after the length field.
Data: data associated with this control frame. The format and length
of this data is controlled by the control frame type.
Control frame processing requirements:
Note that full length control frames (16MB) can be large for
implementations running on resource-limited hardware. In such
cases, implementations MAY limit the maximum length frame
supported. However, all implementations MUST be able to receive
control frames of at least 8192 octets in length.
2.2.2. Data frames
+----------------------------------+
|C| Stream-ID (31bits) |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Data |
+----------------------------------+
Control bit: For data frames this value is always 0.
Stream-ID: A 31-bit value identifying the stream.
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - signifies that this frame represents the last
frame to be transmitted on this stream. See Stream Close
(Section 2.3.7) below.
0x02 = FLAG_COMPRESS - indicates that the data in this frame has
been compressed.
Length: An unsigned 24-bit value representing the number of bytes
after the length field. The total size of a data frame is 8 bytes +
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length. It is valid to have a zero-length data frame.
Data: The variable-length data payload; the length was defined in the
length field.
Data frame processing requirements:
If an endpoint receives a data frame for a stream-id which is not
open and the endpoint has not sent a GOAWAY (Section 2.6.6) frame,
it MUST send issue a stream error (Section 2.4.2) with the error
code INVALID_STREAM for the stream-id.
If the endpoint which created the stream receives a data frame
before receiving a SYN_REPLY on that stream, it is a protocol
error, and the recipient MUST issue a stream error (Section 2.4.2)
with the status code PROTOCOL_ERROR for the stream-id.
Implementors note: If an endpoint receives multiple data frames
for invalid stream-ids, it MAY close the session.
All SPDY endpoints MUST accept compressed data frames.
Compression of data frames is always done using zlib compression.
Each stream initializes and uses its own compression context
dedicated to use within that stream. Endpoints are encouraged to
use application level compression rather than SPDY stream level
compression.
Each SPDY stream sending compressed frames creates its own zlib
context for that stream, and these compression contexts MUST be
distinct from the compression contexts used with SYN_STREAM/
SYN_REPLY/HEADER compression. (Thus, if both endpoints of a
stream are compressing data on the stream, there will be two zlib
contexts, one for sending and one for receiving).
2.3. Streams
Streams are independent sequences of bi-directional data divided into
frames with several properties:
Streams may be created by either the client or server.
Streams optionally carry a set of name/value header pairs.
Streams can concurrently send data interleaved with other streams.
Streams may be cancelled.
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Internet-Draft SPDY Feb 20122.3.1. Stream frames
SPDY defines 3 control frames to manage the lifecycle of a stream:
SYN_STREAM - Open a new stream
SYN_REPLY - Remote acknowledgement of a new, open stream
RST_STREAM - Close a stream
2.3.2. Stream creation
A stream is created by sending a control frame with the type set to
SYN_STREAM (Section 2.6.1). If the server is initiating the stream,
the Stream-ID must be even. If the client is initiating the stream,
the Stream-ID must be odd. 0 is not a valid Stream-ID. Stream-IDs
from each side of the connection must increase monotonically as new
streams are created. E.g. Stream 2 may be created after stream 3,
but stream 7 must not be created after stream 9. Stream IDs do not
wrap: when a client or server cannot create a new stream id without
exceeding a 31 bit value, it MUST NOT create a new stream.
The stream-id MUST increase with each new stream. If an endpoint
receives a SYN_STREAM with a stream id which is less than any
previously received SYN_STREAM, it MUST issue a session error
(Section 2.4.1) with the status PROTOCOL_ERROR.
It is a protocol error to send two SYN_STREAMs with the same
stream-id. If a recipient receives a second SYN_STREAM for the same
stream, it MUST issue a stream error (Section 2.4.2) with the status
code PROTOCOL_ERROR.
Upon receipt of a SYN_STREAM, the recipient can reject the stream by
sending a stream error (Section 2.4.2) with the error code
REFUSED_STREAM. Note, however, that the creating endpoint may have
already sent additional frames for that stream which cannot be
immediately stopped.
Once the stream is created, the creator may immediately send HEADERS
or DATA frames for that stream, without needing to wait for the
recipient to acknowledge.
2.3.2.1. Unidirectional streams
When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag
set, it creates a unidirectional stream which the creating endpoint
can use to send frames, but the receiving endpoint cannot. The
receiving endpoint is implicitly already in the half-closed
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(Section 2.3.6) state.
2.3.2.2. Bidirectional streams
SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are
bidirectional streams. Both endpoints can send data on a bi-
directional stream.
2.3.3. Stream priority
The creator of a stream assigns a priority for that stream. Priority
is represented as an integer from 0 to 7. 0 represents the highest
priority and 7 represents the lowest priority.
The sender and recipient SHOULD use best-effort to process streams in
the order of highest priority to lowest priority.
2.3.4. Stream headers
Streams carry optional sets of name/value pair headers which carry
metadata about the stream. After the stream has been created, and as
long as the sender is not closed (Section 2.3.7) or half-closed
(Section 2.3.6), each side may send HEADERS frame(s) containing the
header data. Header data can be sent in multiple HEADERS frames, and
HEADERS frames may be interleaved with data frames.
2.3.5. Stream data exchange
Once a stream is created, it can be used to send arbitrary amounts of
data. Generally this means that a series of data frames will be sent
on the stream until a frame containing the FLAG_FIN flag is set. The
FLAG_FIN can be set on a SYN_STREAM (Section 2.6.1), SYN_REPLY
(Section 2.6.2), HEADERS (Section 2.6.7) or a DATA (Section 2.2.2)
frame. Once the FLAG_FIN has been sent, the stream is considered to
be half-closed.
2.3.6. Stream half-close
When one side of the stream sends a frame with the FLAG_FIN flag set,
the stream is half-closed from that endpoint. The sender of the
FLAG_FIN MUST NOT send further frames on that stream. When both
sides have half-closed, the stream is closed.
If an endpoint receives a data frame after the stream is half-closed
from the sender (e.g. the endpoint has already received a prior frame
for the stream with the FIN flag set), it MUST send a RST_STREAM to
the sender with the status STREAM_ALREADY_CLOSED.
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Internet-Draft SPDY Feb 20122.3.7. Stream close
There are 3 ways that streams can be terminated:
Normal termination: Normal stream termination occurs when both
sender and recipient have half-closed the stream by sending a
FLAG_FIN.
Abrupt termination: Either the client or server can send a
RST_STREAM control frame at any time. A RST_STREAM contains an
error code to indicate the reason for failure. When a RST_STREAM
is sent from the stream originator, it indicates a failure to
complete the stream and that no further data will be sent on the
stream. When a RST_STREAM is sent from the stream recipient, the
sender, upon receipt, should stop sending any data on the stream.
The stream recipient should be aware that there is a race between
data already in transit from the sender and the time the
RST_STREAM is received. See Stream Error Handling (Section 2.4.2)
TCP connection teardown: If the TCP connection is torn down while
un-closed streams exist, then the endpoint must assume that the
stream was abnormally interrupted and may be incomplete.
If an endpoint receives a data frame after the stream is closed, it
must send a RST_STREAM to the sender with the status PROTOCOL_ERROR.
2.4. Error Handling
The SPDY framing layer has only two types of errors, and they are
always handled consistently. Any reference in this specification to
"issue a session error" refers to Section 2.4.1. Any reference to
"issue a stream error" refers to Section 2.4.2.
2.4.1. Session Error Handling
A session error is any error which prevents further processing of the
framing layer or which corrupts the session compression state. When
a session error occurs, the endpoint encountering the error MUST
first send a GOAWAY (Section 2.6.6) frame with the stream id of most
recently received stream from the remote endpoint, and the error code
for why the session is terminating. After sending the GOAWAY frame,
the endpoint MUST close the TCP connection.
Note that the session compression state is dependent upon both
endpoints always processing all compressed data. If an endpoint
partially processes a frame containing compressed data without
updating compression state properly, future control frames which use
compression will be always be errored. Implementations SHOULD always
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try to process compressed data so that errors which could be handled
as stream errors do not become session errors.
Note that because this GOAWAY is sent during a session error case, it
is possible that the GOAWAY will not be reliably received by the
receiving endpoint. It is a best-effort attempt to communicate with
the remote about why the session is going down.
2.4.2. Stream Error Handling
A stream error is an error related to a specific stream-id which does
not affect processing of other streams at the framing layer. Upon a
stream error, the endpoint MUST send a RST_STREAM (Section 2.6.3)
frame which contains the stream id of the stream where the error
occurred and the error status which caused the error. After sending
the RST_STREAM, the stream is closed to the sending endpoint. After
sending the RST_STREAM, if the sender receives any frames other than
a RST_STREAM for that stream id, it will result in sending additional
RST_STREAM frames. An endpoint MUST NOT send a RST_STREAM in
response to an RST_STREAM, as doing so would lead to RST_STREAM
loops. Sending a RST_STREAM does not cause the SPDY session to be
closed.
If an endpoint has multiple RST_STREAM frames to send in succession
for the same stream-id and the same error code, it MAY coalesce them
into a single RST_STREAM frame. (This can happen if a stream is
closed, but the remote sends multiple data frames. There is no
reason to send a RST_STREAM for each frame in succession).
2.5. Data flow
Because TCP provides a single stream of data on which SPDY
multiplexes multiple logical streams, clients and servers must
intelligently interleave data messages for concurrent sessions.
2.6. Control frame types2.6.1. SYN_STREAM
The SYN_STREAM control frame allows the sender to asynchronously
create a stream between the endpoints. See Stream Creation
(Section 2.3.2)
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+------------------------------------+
|1| version | 1 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
|X| Associated-To-Stream-ID (31bits) |
+------------------------------------+
| Pri|Unused | Slot | |
+-------------------+ |
| Number of Name/Value pairs (int32) | <+
+------------------------------------+ |
| Length of name (int32) | | This section is the "Name/Value
+------------------------------------+ | Header Block", and is compressed.
| Name (string) | |
+------------------------------------+ |
| Length of value (int32) | |
+------------------------------------+ |
| Value (string) | |
+------------------------------------+ |
| (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
(Section 2.3.6) state.
0x02 = FLAG_UNIDIRECTIONAL - a stream created with this flag puts
the recipient in the half-closed (Section 2.3.6) state.
Length: The length is the number of bytes which follow the length
field in the frame. For SYN_STREAM frames, this is 10 bytes plus the
length of the compressed Name/Value block.
Stream-ID: The 31-bit identifier for this stream. This stream-id
will be used in frames which are part of this stream.
Associated-To-Stream-ID: The 31-bit identifier for a stream which
this stream is associated to. If this stream is independent of all
other streams, it should be 0.
Priority: A 3-bit priority (Section 2.3.3) field.
Unused: 5 bits of unused space, reserved for future use.
Slot: An 8 bit unsigned integer specifying the index in the server's
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CREDENTIAL vector of the client certificate to be used for this
request. see CREDENTIAL frame (Section 2.6.9). The value 0 means no
client certificate should be associated with this stream.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 2.6.10).
If an endpoint receives a SYN_STREAM which is larger than the
implementation supports, it MAY send a RST_STREAM with error code
FRAME_TOO_LARGE. All implementations MUST support the minimum size
limits defined in the Control Frames section (Section 2.2.1).
2.6.2. SYN_REPLY
SYN_REPLY indicates the acceptance of a stream creation by the
recipient of a SYN_STREAM frame.
+------------------------------------+
|1| version | 2 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
| Number of Name/Value pairs (int32) | <+
+------------------------------------+ |
| Length of name (int32) | | This section is the "Name/Value
+------------------------------------+ | Header Block", and is compressed.
| Name (string) | |
+------------------------------------+ |
| Length of value (int32) | |
+------------------------------------+ |
| Value (string) | |
+------------------------------------+ |
| (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
(Section 2.3.6) state.
Length: The length is the number of bytes which follow the length
field in the frame. For SYN_REPLY frames, this is 4 bytes plus the
length of the compressed Name/Value block.
Stream-ID: The 31-bit identifier for this stream.
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If an endpoint receives multiple SYN_REPLY frames for the same active
stream ID, it MUST issue a stream error (Section 2.4.2) with the
error code STREAM_IN_USE.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 2.6.10).
If an endpoint receives a SYN_REPLY which is larger than the
implementation supports, it MAY send a RST_STREAM with error code
FRAME_TOO_LARGE. All implementations MUST support the minimum size
limits defined in the Control Frames section (Section 2.2.1).
2.6.3. RST_STREAM
The RST_STREAM frame allows for abnormal termination of a stream.
When sent by the creator of a stream, it indicates the creator wishes
to cancel the stream. When sent by the recipient of a stream, it
indicates an error or that the recipient did not want to accept the
stream, so the stream should be closed.
+----------------------------------+
|1| version | 3 |
+----------------------------------+
| Flags (8) | 8 |
+----------------------------------+
|X| Stream-ID (31bits) |
+----------------------------------+
| Status code |
+----------------------------------+
Flags: Flags related to this frame. RST_STREAM does not define any
flags. This value must be 0.
Length: An unsigned 24-bit value representing the number of bytes
after the length field. For RST_STREAM control frames, this value is
always 8.
Stream-ID: The 31-bit identifier for this stream.
Status code: (32 bits) An indicator for why the stream is being
terminated.The following status codes are defined:
1 - PROTOCOL_ERROR. This is a generic error, and should only be
used if a more specific error is not available.
2 - INVALID_STREAM. This is returned when a frame is received for
a stream which is not active.
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3 - REFUSED_STREAM. Indicates that the stream was refused before
any processing has been done on the stream.
4 - UNSUPPORTED_VERSION. Indicates that the recipient of a stream
does not support the SPDY version requested.
5 - CANCEL. Used by the creator of a stream to indicate that the
stream is no longer needed.
6 - INTERNAL_ERROR. This is a generic error which can be used
when the implementation has internally failed, not due to anything
in the protocol.
7 - FLOW_CONTROL_ERROR. The endpoint detected that its peer
violated the flow control protocol.
8 - STREAM_IN_USE. The endpoint received a SYN_REPLY for a stream
already open.
9 - STREAM_ALREADY_CLOSED. The endpoint received a data or
SYN_REPLY frame for a stream which is half closed.
10 - INVALID_CREDENTIALS. The server received a request for a
resource whose origin does not have valid credentials in the
client certificate vector.
11 - FRAME_TOO_LARGE. The endpoint received a frame which this
implementation could not support. If FRAME_TOO_LARGE is sent for
a SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing
the compressed portion of those frames, then the compression state
will be out-of-sync with the other endpoint. In this case,
senders of FRAME_TOO_LARGE MUST close the session.
Note: 0 is not a valid status code for a RST_STREAM.
After receiving a RST_STREAM on a stream, the recipient must not send
additional frames for that stream, and the stream moves into the
closed state.
2.6.4. SETTINGS
A SETTINGS frame contains a set of id/value pairs for communicating
configuration data about how the two endpoints may communicate.
SETTINGS frames can be sent at any time by either endpoint, are
optionally sent, and are fully asynchronous. When the server is the
sender, the sender can request that configuration data be persisted
by the client across SPDY sessions and returned to the server in
future communications.
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Persistence of SETTINGS ID/Value pairs is done on a per origin/IP
pair (the "origin" is the set of scheme, host, and port from the URI.
See [RFC6454]). That is, when a client connects to a server, and the
server persists settings within the client, the client SHOULD return
the persisted settings on future connections to the same origin AND
IP address and TCP port. Clients MUST NOT request servers to use the
persistence features of the SETTINGS frames, and servers MUST ignore
persistence related flags sent by a client.
+----------------------------------+
|1| version | 4 |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Number of entries |
+----------------------------------+
| ID/Value Pairs |
| ... |
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a SETTINGS message is 4.
Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client
should clear any previously persisted SETTINGS ID/Value pairs. If
this frame contains ID/Value pairs with the
FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear its
existing, persisted settings, and then persist the values with the
flag set which are contained within this frame. Because persistence
is only implemented on the client, this flag can only be used when
the sender is the server.
Length: An unsigned 24-bit value representing the number of bytes
after the length field. The total size of a SETTINGS frame is 8
bytes + length.
Number of entries: A 32-bit value representing the number of ID/value
pairs in this message.
ID: A 32-bit ID number, comprised of 8 bits of flags and 24 bits of
unique ID.
ID.flags:
FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this
SETTINGS frame is requesting that the recipient persist the ID/
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Value and return it in future SETTINGS frames sent from the
sender to this recipient. Because persistence is only
implemented on the client, this flag is only sent by the
server.
FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is
notifying the recipient that this ID/Value pair was previously
sent to the sender by the recipient with the
FLAG_SETTINGS_PERSIST_VALUE, and the sender is returning it.
Because persistence is only implemented on the client, this
flag is only sent by the client.
Defined IDs:
1 - SETTINGS_UPLOAD_BANDWIDTH allows the sender to send its
expected upload bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes
per second that the sender predicts as an expected maximum
upload channel capacity.
2 - SETTINGS_DOWNLOAD_BANDWIDTH allows the sender to send its
expected download bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes
per second that the sender predicts as an expected maximum
download channel capacity.
3 - SETTINGS_ROUND_TRIP_TIME allows the sender to send its
expected round-trip-time on this channel. The round trip time
is defined as the minimum amount of time to send a control
frame from this client to the remote and receive a response.
The value is represented in milliseconds.
4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform
the remote endpoint the maximum number of concurrent streams
which it will allow. By default there is no limit. For
implementors it is recommended that this value be no smaller
than 100.
5 - SETTINGS_CURRENT_CWND allows the sender to inform the
remote endpoint of the current TCP CWND value.
6 - SETTINGS_DOWNLOAD_RETRANS_RATE allows the sender to inform
the remote endpoint the retransmission rate (bytes
retransmitted / total bytes transmitted).
7 - SETTINGS_INITIAL_WINDOW_SIZE allows the sender to inform
the remote endpoint the initial window size (in bytes) for new
streams.
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8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server
to inform the client if the new size of the client certificate
vector.
Value: A 32-bit value.
The message is intentionally extensible for future information which
may improve client-server communications. The sender does not need
to send every type of ID/value. It must only send those for which it
has accurate values to convey. When multiple ID/value pairs are
sent, they should be sent in order of lowest id to highest id. A
single SETTINGS frame MUST not contain multiple values for the same
ID. If the recipient of a SETTINGS frame discovers multiple values
for the same ID, it MUST ignore all values except the first one.
A server may send multiple SETTINGS frames containing different ID/
Value pairs. When the same ID/Value is sent twice, the most recent
value overrides any previously sent values. If the server sends IDs
1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS
frame, and then sends IDs 4 and 5 with the
FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
state on its next SETTINGS frame, it SHOULD send all 5 settings (1,
2, 3, 4, and 5 in this example) to the server.
2.6.5. PING
The PING control frame is a mechanism for measuring a minimal round-
trip time from the sender. It can be sent from the client or the
server. Recipients of a PING frame should send an identical frame to
the sender as soon as possible (if there is other pending data
waiting to be sent, PING should take highest priority). Each ping
sent by a sender should use a unique ID.
+----------------------------------+
|1| version | 6 |
+----------------------------------+
| 0 (flags) | 4 (length) |
+----------------------------------|
| 32-bit ID |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a PING message is 6.
Length: This frame is always 4 bytes long.
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ID: A unique ID for this ping, represented as an unsigned 32 bit
value. When the client initiates a ping, it must use an odd numbered
ID. When the server initiates a ping, it must use an even numbered
ping. Use of odd/even IDs is required in order to avoid accidental
looping on PINGs (where each side initiates an identical PING at the
same time).
Note: If a sender uses all possible PING ids (e.g. has sent all 2^31
possible IDs), it can wrap and start re-using IDs.
If a server receives an even numbered PING which it did not initiate,
it must ignore the PING. If a client receives an odd numbered PING
which it did not initiate, it must ignore the PING.
2.6.6. GOAWAY
The GOAWAY control frame is a mechanism to tell the remote side of
the connection to stop creating streams on this session. It can be
sent from the client or the server. Once sent, the sender will not
respond to any new SYN_STREAMs on this session. Recipients of a
GOAWAY frame must not send additional streams on this session,
although a new session can be established for new streams. The
purpose of this message is to allow an endpoint to gracefully stop
accepting new streams (perhaps for a reboot or maintenance), while
still finishing processing of previously established streams.
There is an inherent race condition between an endpoint sending
SYN_STREAMs and the remote sending a GOAWAY message. To deal with
this case, the GOAWAY contains a last-stream-id indicating the
stream-id of the last stream which was created on the sending
endpoint in this session. If the receiver of the GOAWAY sent new
SYN_STREAMs for sessions after this last-stream-id, they were not
processed by the server and the receiver may treat the stream as
though it had never been created at all (hence the receiver may want
to re-create the stream later on a new session).
Endpoints should always send a GOAWAY message before closing a
connection so that the remote can know whether a stream has been
partially processed or not. (For example, if an HTTP client sends a
POST at the same time that a server closes a connection, the client
cannot know if the server started to process that POST request if the
server does not send a GOAWAY frame to indicate where it stopped
working).
After sending a GOAWAY message, the sender must ignore all SYN_STREAM
frames for new streams.
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+----------------------------------+
|1| version | 7 |
+----------------------------------+
| 0 (flags) | 8 (length) |
+----------------------------------|
|X| Last-good-stream-ID (31 bits) |
+----------------------------------+
| Status code |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a GOAWAY message is 7.
Length: This frame is always 8 bytes long.
Last-good-stream-Id: The last stream id which was replied to (with
either a SYN_REPLY or RST_STREAM) by the sender of the GOAWAY
message. If no streams were replied to, this value MUST be 0.
Status: The reason for closing the session.
0 - OK. This is a normal session teardown.
1 - PROTOCOL_ERROR. This is a generic error, and should only be
used if a more specific error is not available.
11 - INTERNAL_ERROR. This is a generic error which can be used
when the implementation has internally failed, not due to anything
in the protocol.
2.6.7. HEADERS
The HEADERS frame augments a stream with additional headers. It may
be optionally sent on an existing stream at any time. Specific
application of the headers in this frame is application-dependent.
The name/value header block within this frame is compressed.
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+------------------------------------+
|1| version | 8 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
| Number of Name/Value pairs (int32) | <+
+------------------------------------+ |
| Length of name (int32) | | This section is the "Name/Value
+------------------------------------+ | Header Block", and is compressed.
| Name (string) | |
+------------------------------------+ |
| Length of value (int32) | |
+------------------------------------+ |
| Value (string) | |
+------------------------------------+ |
| (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
(Section 2.3.6) state.
Length: An unsigned 24 bit value representing the number of bytes
after the length field. The minimum length of the length field is 4
(when the number of name value pairs is 0).
Stream-ID: The stream this HEADERS block is associated with.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 2.6.10).
2.6.8. WINDOW_UPDATE
The WINDOW_UPDATE control frame is used to implement per stream flow
control in SPDY. Flow control in SPDY is per hop, that is, only
between the two endpoints of a SPDY connection. If there are one or
more intermediaries between the client and the origin server, flow
control signals are not explicitly forwarded by the intermediaries.
(However, throttling of data transfer by any recipient may have the
effect of indirectly propagating flow control information upstream
back to the original sender.) Flow control only applies to the data
portion of data frames. Recipients must buffer all control frames.
If a recipient fails to buffer an entire control frame, it MUST issue
a stream error (Section 2.4.2) with the status code
FLOW_CONTROL_ERROR for the stream.
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Flow control in SPDY is implemented by a data transfer window kept by
the sender of each stream. The data transfer window is a simple
uint32 that indicates how many bytes of data the sender can transmit.
After a stream is created, but before any data frames have been
transmitted, the sender begins with the initial window size. This
window size is a measure of the buffering capability of the
recipient. The sender must not send a data frame with data length
greater than the transfer window size. After sending each data
frame, the sender decrements its transfer window size by the amount
of data transmitted. When the window size becomes less than or equal
to 0, the sender must pause transmitting data frames. At the other
end of the stream, the recipient sends a WINDOW_UPDATE control back
to notify the sender that it has consumed some data and freed up
buffer space to receive more data.
+----------------------------------+
|1| version | 9 |
+----------------------------------+
| 0 (flags) | 8 (length) |
+----------------------------------+
|X| Stream-ID (31-bits) |
+----------------------------------+
|X| Delta-Window-Size (31-bits) |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a WINDOW_UPDATE message is 9.
Length: The length field is always 8 for this frame (there are 8
bytes after the length field).
Stream-ID: The stream ID that this WINDOW_UPDATE control frame is
for.
Delta-Window-Size: The additional number of bytes that the sender can
transmit in addition to existing remaining window size. The legal
range for this field is 1 to 2^31 - 1 (0x7fffffff) bytes.
The window size as kept by the sender must never exceed 2^31
(although it can become negative in one special case). If a sender
receives a WINDOW_UPDATE that causes the its window size to exceed
this limit, it must send RST_STREAM with status code
FLOW_CONTROL_ERROR to terminate the stream.
When a SPDY connection is first established, the default initial
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window size for all streams is 64KB. An endpoint can use the
SETTINGS control frame to adjust the initial window size for the
connection. That is, its peer can start out using the 64KB default
initial window size when sending data frames before receiving the
SETTINGS. Because SETTINGS is asynchronous, there may be a race
condition if the recipient wants to decrease the initial window size,
but its peer immediately sends 64KB on the creation of a new
connection, before waiting for the SETTINGS to arrive. This is one
case where the window size kept by the sender will become negative.
Once the sender detects this condition, it must stop sending data
frames and wait for the recipient to catch up. The recipient has two
choices:
immediately send RST_STREAM with FLOW_CONTROL_ERROR status code.
allow the head of line blocking (as there is only one stream for
the session and the amount of data in flight is bounded by the
default initial window size), and send WINDOW_UPDATE as it
consumes data.
In the case of option 2, both sides must compute the window size
based on the initial window size in the SETTINGS. For example, if
the recipient sets the initial window size to be 16KB, and the sender
sends 64KB immediately on connection establishment, the sender will
discover its window size is -48KB on receipt of the SETTINGS. As the
recipient consumes the first 16KB, it must send a WINDOW_UPDATE of
16KB back to the sender. This interaction continues until the
sender's window size becomes positive again, and it can resume
transmitting data frames.
After the recipient reads in a data frame with FLAG_FIN that marks
the end of the data stream, it should not send WINDOW_UPDATE frames
as it consumes the last data frame. A sender should ignore all the
WINDOW_UPDATE frames associated with the stream after it send the
last frame for the stream.
The data frames from the sender and the WINDOW_UPDATE frames from the
recipient are completely asynchronous with respect to each other.
This property allows a recipient to aggressively update the window
size kept by the sender to prevent the stream from stalling.
2.6.9. CREDENTIAL
The CREDENTIAL control frame is used by the client to send additional
client certificates to the server. A SPDY client may decide to send
requests for resources from different origins on the same SPDY
session if it decides that that server handles both origins. For
example if the IP address associated with both hostnames matches and
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the SSL server certificate presented in the initial handshake is
valid for both hostnames. However, because the SSL connection can
contain at most one client certificate, the client needs a mechanism
to send additional client certificates to the server.
The server is required to maintain a vector of client certificates
associated with a SPDY session. When the client needs to send a
client certificate to the server, it will send a CREDENTIAL frame
that specifies the index of the slot in which to store the
certificate as well as proof that the client posesses the
corresponding private key. The initial size of this vector must be
8. If the client provides a client certificate during the first TLS
handshake, the contents of this certificate must be copied into the
first slot (index 1) in the CREDENTIAL vector, though it may be
overwritten by subsequent CREDENTIAL frames. The server must
exclusively use the CREDNETIAL vector when evaluating the client
certificates associated with an origin. The server may change the
size of this vector by sending a SETTINGS frame with the setting
SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE value specified. In the
event that the new size is smaller than the current size, truncation
occurs preserving lower-index slots as possible.
TLS renegotiation with client authentication is incompatible with
SPDY given the multiplexed nature of SPDY. Specifically, imagine
that the client has 2 requests outstanding to the server for two
different pages (in different tabs). When the renegotiation + client
certificate request comes in, the browser is unable to determine
which resource triggered the client certificate request, in order to
prompt the user accordingly.
+----------------------------------+
|1|000000000000001|0000000000001011|
+----------------------------------+
| flags (8) | Length (24 bits) |
+----------------------------------+
| Slot (16 bits) | |
+-----------------+ |
| Proof Length (32 bits) |
+----------------------------------+
| Proof |
+----------------------------------+ <+
| Certificate Length (32 bits) | |
+----------------------------------+ | Repeated until end of frame
| Certificate | |
+----------------------------------+ <+
Slot: The index in the server's client certificate vector where this
certificate should be stored. If there is already a certificate
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stored at this index, it will be overwritten. The index is one
based, not zero based; zero is an invalid slot index.
Proof: Cryptographic proof that the client has possession of the
private key associated with the certificate. The format is a TLS
digitally-signed element
(http://tools.ietf.org/html/rfc5246#section-4.7). The signature
algorithm must be the same as that used in the CertificateVerify
message. However, since the MD5+SHA1 signature type used in TLS 1.0
connections can not be correctly encoded in a digitally-signed
element, SHA1 must be used when MD5+SHA1 was used in the SSL
connection. The signature is calculated over a 32 byte TLS extractor
value (http://tools.ietf.org/html/rfc5705) with a label of "EXPORTER
SPDY certificate proof" using the empty string as context. ForRSA
certificates the signature would be a PKCS#1 v1.5 signature. For
ECDSA, it would be an ECDSA-Sig-Value
(http://tools.ietf.org/html/rfc5480#appendix-A). For a 1024-bit RSA
key, the CREDENTIAL message would be ~500 bytes.
Certificate: The certificate chain, starting with the leaf
certificate. Each certificate must be encoded as a 32 bit length,
followed by a DER encoded certificate. The certificate must be of
the same type (RSA, ECDSA, etc) as the client certificate associated
with the SSL connection.
If the server receives a request for a resource with unacceptable
credential (either missing or invalid), it must reply with a
RST_STREAM frame with the status code INVALID_CREDENTIALS. Upon
receipt of a RST_STREAM frame with INVALID_CREDENTIALS, the client
should initiate a new stream directly to the requested origin and
resend the request. Note, SPDY does not allow the server to request
different client authentication for different resources in the same
origin.
If the server receives an invalid CREDENTIAL frame, it MUST respond
with a GOAWAY frame and shutdown the session.
2.6.10. Name/Value Header Block
The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and
HEADERS control frames, and shares a common format:
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+------------------------------------+
| Number of Name/Value pairs (int32) |
+------------------------------------+
| Length of name (int32) |
+------------------------------------+
| Name (string) |
+------------------------------------+
| Length of value (int32) |
+------------------------------------+
| Value (string) |
+------------------------------------+
| (repeats) |
Number of Name/Value pairs: The number of repeating name/value pairs
following this field.
List of Name/Value pairs:
Length of Name: a 32-bit value containing the number of octets in
the name field. Note that in practice, this length must not
exceed 2^24, as that is the maximum size of a SPDY frame.
Name: 0 or more octets, 8-bit sequences of data, excluding 0.
Length of Value: a 32-bit value containing the number of octets in
the value field. Note that in practice, this length must not
exceed 2^24, as that is the maximum size of a SPDY frame.
Value: 0 or more octets, 8-bit sequences of data, excluding 0.
Each header name must have at least one value. Header names are
encoded using the US-ASCII character set [ASCII] and must be all
lower case. The length of each name must be greater than zero. A
recipient of a zero-length name MUST issue a stream error
(Section 2.4.2) with the status code PROTOCOL_ERROR for the
stream-id.
Duplicate header names are not allowed. To send two identically
named headers, send a header with two values, where the values are
separated by a single NUL (0) byte. A header value can either be
empty (e.g. the length is zero) or it can contain multiple, NUL-
separated values, each with length greater than zero. The value
never starts nor ends with a NUL character. Recipients of illegal
value fields MUST issue a stream error (Section 2.4.2) with the
status code PROTOCOL_ERROR for the stream-id.
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The entire contents of the name/value header block is compressed
using zlib. There is a single zlib stream for all name value pairs
in one direction on a connection. SPDY uses a SYNC_FLUSH between
each compressed frame.
Implementation notes: the compression engine can be tuned to favor
speed or size. Optimizing for size increases memory use and CPU
consumption. Because header blocks are generally small, implementors
may want to reduce the window-size of the compression engine from the
default 15bits (a 32KB window) to more like 11bits (a 2KB window).
The exact setting is chosen by the compressor, the decompressor will
work with any setting.
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SPDY is intended to be as compatible as possible with current web-
based applications. This means that, from the perspective of the
server business logic or application API, the features of HTTP are
unchanged. To achieve this, all of the application request and
response header semantics are preserved, although the syntax of
conveying those semantics has changed. Thus, the rules from the
HTTP/1.1 specification in RFC2616 [RFC2616] apply with the changes in
the sections below.
3.1. Connection Management
Clients SHOULD NOT open more than one SPDY session to a given origin
[RFC6454] concurrently.
Note that it is possible for one SPDY session to be finishing (e.g. a
GOAWAY message has been sent, but not all streams have finished),
while another SPDY session is starting.
3.1.1. Use of GOAWAY
SPDY provides a GOAWAY message which can be used when closing a
connection from either the client or server. Without a server GOAWAY
message, HTTP has a race condition where the client sends a request
(a new SYN_STREAM) just as the server is closing the connection, and
the client cannot know if the server received the stream or not. By
using the last-stream-id in the GOAWAY, servers can indicate to the
client if a request was processed or not.
Note that some servers will choose to send the GOAWAY and immediately
terminate the connection without waiting for active streams to
finish. The client will be able to determine this because SPDY
streams are determinstically closed. This abrupt termination will
force the client to heuristically decide whether to retry the pending
requests. Clients always need to be capable of dealing with this
case because they must deal with accidental connection termination
cases, which are the same as the server never having sent a GOAWAY.
More sophisticated servers will use GOAWAY to implement a graceful
teardown. They will send the GOAWAY and provide some time for the
active streams to finish before terminating the connection.
If a SPDY client closes the connection, it should also send a GOAWAY
message. This allows the server to know if any server-push streams
were received by the client.
If the endpoint closing the connection has not received any
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SYN_STREAMs from the remote, the GOAWAY will contain a last-stream-id
of 0.
3.2. HTTP Request/Response3.2.1. Request
The client initiates a request by sending a SYN_STREAM frame. For
requests which do not contain a body, the SYN_STREAM frame MUST set
the FLAG_FIN, indicating that the client intends to send no further
data on this stream. For requests which do contain a body, the
SYN_STREAM will not contain the FLAG_FIN, and the body will follow
the SYN_STREAM in a series of DATA frames. The last DATA frame will
set the FLAG_FIN to indicate the end of the body.
The SYN_STREAM Name/Value section will contain all of the HTTP
headers which are associated with an HTTP request. The header block
in SPDY is mostly unchanged from today's HTTP header block, with the
following differences:
The first line of the request is unfolded into name/value pairs
like other HTTP headers and MUST be present:
":method" - the HTTP method for this request (e.g. "GET",
"POST", "HEAD", etc)
":path" - the url-path for this url with "/" prefixed. (See
RFC1738 [RFC1738]). For example, for
"http://www.google.com/search?q=dogs" the path would be
"/search?q=dogs".
":version" - the HTTP version of this request (e.g.
"HTTP/1.1")
In addition, the following two name/value pairs must also be
present in every request:
":host" - the hostport (See RFC1738 [RFC1738]) portion of the
URL for this request (e.g. "www.google.com:1234"). This header
is the same as the HTTP 'Host' header.
":scheme" - the scheme portion of the URL for this request
(e.g. "https"))
Header names are all lowercase.
The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
Encoding headers are not valid and MUST not be sent.
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User-agents MUST support gzip compression. Regardless of the
Accept-Encoding sent by the user-agent, the server may always send
content encoded with gzip or deflate encoding.
If a server receives a request where the sum of the data frame
payload lengths does not equal the size of the Content-Length
header, the server MUST return a 400 (Bad Request) error.
POST-specific changes:
Although POSTs are inherently chunked, POST requests SHOULD
also be accompanied by a Content-Length header. There are two
reasons for this: First, it assists with upload progress meters
for an improved user experience. But second, we know from
early versions of SPDY that failure to send a content length
header is incompatible with many existing HTTP server
implementations. Existing user-agents do not omit the Content-
Length header, and server implementations have come to depend
upon this.
The user-agent is free to prioritize requests as it sees fit. If the
user-agent cannot make progress without receiving a resource, it
should attempt to raise the priority of that resource. Resources
such as images, SHOULD generally use the lowest priority.
If a client sends a SYN_STREAM without all of the method, host, path,
scheme, and version headers, the server MUST reply with a HTTP 400
Bad Request reply.
3.2.2. Response
The server responds to a client request with a SYN_REPLY frame.
Symmetric to the client's upload stream, server will send data after
the SYN_REPLY frame via a series of DATA frames, and the last data
frame will contain the FLAG_FIN to indicate successful end-of-stream.
If a response (like a 202 or 204 response) contains no body, the
SYN_REPLY frame may contain the FLAG_FIN flag to indicate no further
data will be sent on the stream.
The response status line is unfolded into name/value pairs like
other HTTP headers and must be present:
":status" - The HTTP response status code (e.g. "200" or "200
OK")
":version" - The HTTP response version (e.g. "HTTP/1.1")
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All header names must be lowercase.
The Connection, Keep-Alive, Proxy-Connection, and Transfer-
Encoding headers are not valid and MUST not be sent.
Responses MAY be accompanied by a Content-Length header for
advisory purposes. (e.g. for UI progress meters)
If a client receives a response where the sum of the data frame
payload lengths does not equal the size of the Content-Length
header, the client MUST ignore the content length header.
If a client receives a SYN_REPLY without a status or without a
version header, the client must reply with a RST_STREAM frame
indicating a PROTOCOL ERROR.
3.2.3. Authentication
When a client sends a request to an origin server that requires
authentication, the server can reply with a "401 Unauthorized"
response, and include a WWW-Authenticate challenge header that
defines the authentication scheme to be used. The client then
retries the request with an Authorization header appropriate to the
specified authentication scheme.
There are four options for proxy authentication, Basic, Digest, NTLM
and Negotiate (SPNEGO). The first two options were defined in
RFC2617 [RFC2617], and are stateless. The second two options were
developed by Microsoft and specified in RFC4559 [RFC4559], and are
stateful; otherwise known as multi-round authentication, or
connection authentication.
3.2.3.1. Stateless Authentication
Stateless Authentication over SPDY is identical to how it is
performed over HTTP. If multiple SPDY streams are concurrently sent
to a single server, each will authenticate independently, similar to
how two HTTP connections would independently authenticate to a proxy
server.
3.2.3.2. Stateful Authentication
Unfortunately, the stateful authentication mechanisms were
implemented and defined in a such a way that directly violates
RFC2617 - they do not include a "realm" as part of the request. This
is problematic in SPDY because it makes it impossible for a client to
disambiguate two concurrent server authentication challenges.
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To deal with this case, SPDY servers using Stateful Authentication
MUST implement one of two changes:
Servers can add a "realm=<desired realm>" header so that the two
authentication requests can be disambiguated and run concurrently.
Unfortunately, given how these mechanisms work, this is probably
not practical.
Upon sending the first stateful challenge response, the server
MUST buffer and defer all further frames which are not part of
completing the challenge until the challenge has completed.
Completing the authentication challenge may take multiple round
trips. Once the client receives a "401 Authenticate" response for
a stateful authentication type, it MUST stop sending new requests
to the server until the authentication has completed by receiving
a non-401 response on at least one stream.
3.3. Server Push Transactions
SPDY enables a server to send multiple replies to a client for a
single request. The rationale for this feature is that sometimes a
server knows that it will need to send multiple resources in response
to a single request. Without server push features, the client must
first download the primary resource, then discover the secondary
resource(s), and request them. Pushing of resources avoids the
round-trip delay, but also creates a potential race where a server
can be pushing content which a user-agent is in the process of
requesting. The following mechanics attempt to prevent the race
condition while enabling the performance benefit.
Browsers receiving a pushed response MUST validate that the server is
authorized to push the URL using the browser same-origin [RFC6454]
policy. For example, a SPDY connection to www.foo.com is generally
not permitted to push a response for www.evil.com.
If the browser accepts a pushed response (e.g. it does not send a
RST_STREAM), the browser MUST attempt to cache the pushed response in
same way that it would cache any other response. This means
validating the response headers and inserting into the disk cache.
Because pushed responses have no request, they have no request
headers associated with them. At the framing layer, SPDY pushed
streams contain an "associated-stream-id" which indicates the
requested stream for which the pushed stream is related. The pushed
stream inherits all of the headers from the associated-stream-id with
the exception of ":host", ":scheme", and ":path", which are provided
as part of the pushed response stream headers. The browser MUST
store these inherited and implied request headers with the cached
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resource.
Implementation note: With server push, it is theoretically possible
for servers to push unreasonable amounts of content or resources to
the user-agent. Browsers MUST implement throttles to protect against
unreasonable push attacks.
3.3.1. Server implementation
When the server intends to push a resource to the user-agent, it
opens a new stream by sending a unidirectional SYN_STREAM. The
SYN_STREAM MUST include an Associated-To-Stream-ID, and MUST set the
FLAG_UNIDIRECTIONAL flag. The SYN_STREAM MUST include headers for
":scheme", ":host", ":path", which represent the URL for the resource
being pushed. Subsequent headers may follow in HEADERS frames. The
purpose of the association is so that the user-agent can
differentiate which request induced the pushed stream; without it, if
the user-agent had two tabs open to the same page, each pushing
unique content under a fixed URL, the user-agent would not be able to
differentiate the requests.
The Associated-To-Stream-ID must be the ID of an existing, open
stream. The reason for this restriction is to have a clear endpoint
for pushed content. If the user-agent requested a resource on stream
11, the server replies on stream 11. It can push any number of
additional streams to the client before sending a FLAG_FIN on stream
11. However, once the originating stream is closed no further push
streams may be associated with it. The pushed streams do not need to
be closed (FIN set) before the originating stream is closed, they
only need to be created before the originating stream closes.
It is illegal for a server to push a resource with the Associated-To-
Stream-ID of 0.
To minimize race conditions with the client, the SYN_STREAM for the
pushed resources MUST be sent prior to sending any content which
could allow the client to discover the pushed resource and request
it.
The server MUST only push resources which would have been returned
from a GET request.
Note: If the server does not have all of the Name/Value Response
headers available at the time it issues the HEADERS frame for the
pushed resource, it may later use an additional HEADERS frame to
augment the name/value pairs to be associated with the pushed stream.
The subsequent HEADERS frame(s) must not contain a header for
':host', ':scheme', or ':path' (e.g. the server can't change the
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identity of the resource to be pushed). The HEADERS frame must not
contain duplicate headers with a previously sent HEADERS frame. The
server must send a HEADERS frame including the scheme/host/port
headers before sending any data frames on the stream.
3.3.2. Client implementation
When fetching a resource the client has 3 possibilities:
the resource is not being pushed
the resource is being pushed, but the data has not yet arrived
the resource is being pushed, and the data has started to arrive
When a SYN_STREAM and HEADERS frame which contains an Associated-To-
Stream-ID is received, the client must not issue GET requests for the
resource in the pushed stream, and instead wait for the pushed stream
to arrive.
If a client receives a server push stream with stream-id 0, it MUST
issue a session error (Section 2.4.1) with the status code
PROTOCOL_ERROR.
When a client receives a SYN_STREAM from the server without a the
':host', ':scheme', and ':path' headers in the Name/Value section, it
MUST reply with a RST_STREAM with error code HTTP_PROTOCOL_ERROR.
To cancel individual server push streams, the client can issue a
stream error (Section 2.4.2) with error code CANCEL. Upon receipt,
the server MUST stop sending on this stream immediately (this is an
Abrupt termination).
To cancel all server push streams related to a request, the client
may issue a stream error (Section 2.4.2) with error code CANCEL on
the associated-stream-id. By cancelling that stream, the server MUST
immediately stop sending frames for any streams with
in-association-to for the original stream.
If the server sends a HEADER frame containing duplicate headers with
a previous HEADERS frame for the same stream, the client must issue a
stream error (Section 2.4.2) with error code PROTOCOL ERROR.
If the server sends a HEADERS frame after sending a data frame for
the same stream, the client MAY ignore the HEADERS frame. Ignoring
the HEADERS frame after a data frame prevents handling of HTTP's
trailing headers
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40).
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Internet-Draft SPDY Feb 20124. Design Rationale and Notes
Authors' notes: The notes in this section have no bearing on the SPDY
protocol as specified within this document, and none of these notes
should be considered authoritative about how the protocol works.
However, these notes may prove useful in future debates about how to
resolve protocol ambiguities or how to evolve the protocol going
forward. They may be removed before the final draft.
4.1. Separation of Framing Layer and Application Layer
Readers may note that this specification sometimes blends the framing
layer (Section 2) with requirements of a specific application - HTTP
(Section 3). This is reflected in the request/response nature of the
streams, the definition of the HEADERS and compression contexts which
are very similar to HTTP, and other areas as well.
This blending is intentional - the primary goal of this protocol is
to create a low-latency protocol for use with HTTP. Isolating the
two layers is convenient for description of the protocol and how it
relates to existing HTTP implementations. However, the ability to
reuse the SPDY framing layer is a non goal.
4.2. Error handling - Framing Layer
Error handling at the SPDY layer splits errors into two groups: Those
that affect an individual SPDY stream, and those that do not.
When an error is confined to a single stream, but general framing is
in tact, SPDY attempts to use the RST_STREAM as a mechanism to
invalidate the stream but move forward without aborting the
connection altogether.
For errors occuring outside of a single stream context, SPDY assumes
the entire session is hosed. In this case, the endpoint detecting
the error should initiate a connection close.
4.3. One Connection Per Domain
SPDY attempts to use fewer connections than other protocols have
traditionally used. The rationale for this behavior is because it is
very difficult to provide a consistent level of service (e.g. TCP
slow-start), prioritization, or optimal compression when the client
is connecting to the server through multiple channels.
Through lab measurements, we have seen consistent latency benefits by
using fewer connections from the client. The overall number of
packets sent by SPDY can be as much as 40% less than HTTP. Handling
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large numbers of concurrent connections on the server also does
become a scalability problem, and SPDY reduces this load.
The use of multiple connections is not without benefit, however.
Because SPDY multiplexes multiple, independent streams onto a single
stream, it creates a potential for head-of-line blocking problems at
the transport level. In tests so far, the negative effects of head-
of-line blocking (especially in the presence of packet loss) is
outweighed by the benefits of compression and prioritization.
4.4. Fixed vs Variable Length Fields
SPDY favors use of fixed length 32bit fields in cases where smaller,
variable length encodings could have been used. To some, this seems
like a tragic waste of bandwidth. SPDY choses the simple encoding
for speed and simplicity.
The goal of SPDY is to reduce latency on the network. The overhead
of SPDY frames is generally quite low. Each data frame is only an 8
byte overhead for a 1452 byte payload (~0.6%). At the time of this
writing, bandwidth is already plentiful, and there is a strong trend
indicating that bandwidth will continue to increase. With an average
worldwide bandwidth of 1Mbps, and assuming that a variable length
encoding could reduce the overhead by 50%, the latency saved by using
a variable length encoding would be less than 100 nanoseconds. More
interesting are the effects when the larger encodings force a packet
boundary, in which case a round-trip could be induced. However, by
addressing other aspects of SPDY and TCP interactions, we believe
this is completely mitigated.
4.5. Compression Context(s)
When isolating the compression contexts used for communicating with
multiple origins, we had a few choices to make. We could have
maintained a map (or list) of compression contexts usable for each
origin. The basic case is easy - each HEADERS frame would need to
identify the context to use for that frame. However, compression
contexts are not cheap, so the lifecycle of each context would need
to be bounded. For proxy servers, where we could churn through many
contexts, this would be a concern. We considered using a static set
of contexts, say 16 of them, which would bound the memory use. We
also considered dynamic contexts, which could be created on the fly,
and would need to be subsequently destroyed. All of these are
complicated, and ultimately we decided that such a mechanism creates
too many problems to solve.
Alternatively, we've chosen the simple approach, which is to simply
provide a flag for resetting the compression context. For the common
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case (no proxy), this fine because most requests are to the same
origin and we never need to reset the context. For cases where we
are using two different origins over a single SPDY session, we simply
reset the compression state between each transition.
4.6. Unidirectional streams
Many readers notice that unidirectional streams are both a bit
confusing in concept and also somewhat redundant. If the recipient
of a stream doesn't wish to send data on a stream, it could simply
send a SYN_REPLY with the FLAG_FIN bit set. The FLAG_UNIDIRECTIONAL
is, therefore, not necessary.
It is true that we don't need the UNIDIRECTIONAL markings. It is
added because it avoids the recipient of pushed streams from needing
to send a set of empty frames (e.g. the SYN_STREAM w/ FLAG_FIN) which
otherwise serve no purpose.
4.7. Data Compression
Generic compression of data portion of the streams (as opposed to
compression of the headers) without knowing the content of the stream
is redundant. There is no value in compressing a stream which is
already compressed. Because of this, SPDY does allow data
compression to be optional. We included it because study of existing
websites shows that many sites are not using compression as they
should, and users suffer because of it. We wanted a mechanism where,
at the SPDY layer, site administrators could simply force compression
- it is better to compress twice than to not compress.
Overall, however, with this feature being optional and sometimes
redundant, it is unclear if it is useful at all. We will likely
remove it from the specification.
4.8. Server Push
A subtle but important point is that server push streams must be
declared before the associated stream is closed. The reason for this
is so that proxies have a lifetime for which they can discard
information about previous streams. If a pushed stream could
associate itself with an already-closed stream, then endpoints would
not have a specific lifecycle for when they could disavow knowledge
of the streams which went before.
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Internet-Draft SPDY Feb 20125. Security Considerations5.1. Use of Same-origin constraints
This specification uses the same-origin policy [RFC6454] in all cases
where verification of content is required.
5.2. HTTP Headers and SPDY Headers
At the application level, HTTP uses name/value pairs in its headers.
Because SPDY merges the existing HTTP headers with SPDY headers,
there is a possibility that some HTTP applications already use a
particular header name. To avoid any conflicts, all headers
introduced for layering HTTP over SPDY are prefixed with ":". ":" is
not a valid sequence in HTTP header naming, preventing any possible
conflict.
5.3. Cross-Protocol Attacks
By utilizing TLS, we believe that SPDY introduces no new cross-
protocol attacks. TLS encrypts the contents of all transmission
(except the handshake itself), making it difficult for attackers to
control the data which could be used in a cross-protocol attack.
5.4. Server Push Implicit Headers
Pushed resources do not have an associated request. In order for
existing HTTP cache control validations (such as the Vary header) to
work, however, all cached resources must have a set of request
headers. For this reason, browsers MUST be careful to inherit
request headers from the associated stream for the push. This
includes the 'Cookie' header.
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Internet-Draft SPDY Feb 20126. Privacy Considerations6.1. Long Lived Connections
SPDY aims to keep connections open longer between clients and servers
in order to reduce the latency when a user makes a request. The
maintenance of these connections over time could be used to expose
private information. For example, a user using a browser hours after
the previous user stopped using that browser may be able to learn
about what the previous user was doing. This is a problem with HTTP
in its current form as well, however the short lived connections make
it less of a risk.
6.2. SETTINGS frame
The SPDY SETTINGS frame allows servers to store out-of-band
transmitted information about the communication between client and
server on the client. Although this is intended only to be used to
reduce latency, renegade servers could use it as a mechanism to store
identifying information about the client in future requests.
Clients implementing privacy modes, such as Google Chrome's
"incognito mode", may wish to disable client-persisted SETTINGS
storage.
Clients MUST clear persisted SETTINGS information when clearing the
cookies.
TODO: Put range maximums on each type of setting to limit
inappropriate uses.
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Internet-Draft SPDY Feb 20127. Incompatibilities with SPDY draft #2
Here is a list of the major changes between this draft and draft #2.
Addition of flow control
Increased 16 bit length fields in SYN_STREAM and SYN_REPLY to 32
bits.
Changed definition of compression for DATA frames
Updated compression dictionary
Fixed off-by-one on the compression dictionary for headers
Increased priority field from 2bits to 3bits.
Removed NOOP frame
Split the request "url" into "scheme", "host", and "path"
Added the requirement that POSTs contain content-length.
Removed wasted 16bits of unused space from the end of the
SYN_REPLY and HEADERS frames.
Fixed bug: Priorities were described backward (0 was lowest
instead of highest).
Fixed bug: Name/Value header counts were duplicated in both the
Name Value header block and also the containing frame.
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